JOHANNESBURG/NAIROBI, Sept 19 (Reuters) - On June 29, Maman Sidikou, head of the U.N. peacekeeping mission in Congo, received a cable from headquarters in New York in which his bosses laid out in no uncertain terms that the world's largest peacekeeping mission had to make cuts, and fast.

Facing an eight percent, or $93 million, budget cut for 2017/18, Sidikou was told to revise staffing, slash fuel costs by 10 percent and streamline aircraft use - all without compromising the mission's mandate, according to the cable seen by Reuters.

The mission in Democratic Republic of Congo, known as MONUSCO, must work out how to juggle those demands with the need to respond to a growing political and humanitarian crisis in the central African giant - and it is not alone.

Belt-tightening at MONUSCO, which has about 18,000 uniformed personnel, is part of a broader push by the United States, the biggest U.N. contributor, to cut costs. In June, the 193 U.N. member states agreed to a total $600 million in cuts to more than a dozen missions for the year ending June 30, 2018.

U.S. Ambassador to the United Nations Nikki Haley said at the time: "We're only getting started."

On Wednesday the 15-member U.N. Security Council will discuss peacekeeping reform during the annual gathering of world leaders.

Diplomats said the council was due to adopt a resolution pushing for improved accountability, transparency and effectiveness and to make peacekeepers more flexible. Critics worry that harsh cuts could harm operations in volatile African states.

"My intention is to do everything to preserve the integrity of the peacekeeping missions, but, of course, to do also everything possible to make it in the most effective and cost-effective way," U.N. Secretary-General Antonio Guterres told reporters last week.

THORNY ISSUES

The United Nations, which has spent $18 billion on peacekeeping in Congo since the mission began in 1999, says reforms are bearing fruit.

But analysts and some U.N. insiders say progress is hampered by administrators in New York dodging thorny issues like confusion over the mission's priorities and a culture that appears to protect senior, well-paid officials at all costs.

In July, for example, Safia Boly became deputy head of the MONUSCO division administering the mission's downsizing, despite the fact that a year earlier, a U.N. tribunal ruled she had abused her authority and disregarded rules at an office in Uganda where she served as operations manager.

"Put very simply, and in other words, who was protecting Ms. Boly and why?" the tribunal wrote in its judgment.

Boly did not respond to an emailed request for comment. U.N. spokesman Farhan Haq said the affair was closed and that it was not appropriate to comment on a confidential internal process.

"This sends a signal that U.N. headquarters is not really interested in reform and cleaning up the system," said a Western diplomat about Boly's case.

In another case, members of an engineering unit in Goma allegedly demanded hundreds of dollars from locals in return for help to be recruited for temporary work, according to a U.N. document seen by Reuters.

U.S. President Donald Trump wants to cap the U.S. share of the $7.3 billion peacekeeping bill at 25 percent, down from 28.5 percent, a level he says is "unfair".

At a meeting on U.N. reform on Monday, Trump said the world body was hamstrung by "bureaucracy and mismanagement". nL2N1LZ0SR]

MORE TROOPS NEEDED?

A peacekeeping mission in Ivory Coast has closed and troop levels in Sudan's Darfur region are due to be halved, but U.N. officials say they need more, not fewer, blue helmets in hot spots like Mali, Central African Republic and South Sudan.

"Peacekeeping reform is essential, and the U.S. should lead in demanding better performance and accountability. But that will not be achieved by crippling the ability of U.N. troops and civilian personnel to operate where they're needed most," said Matt Wells, senior crisis adviser at Amnesty International.

MONUSCO's military leaders want to replace underperforming units, long accused of passivity in protecting civilians, with more capable troops. But they are held back by long-standing reluctance among member states to risk soldiers' lives in distant conflicts.

In a July 15 cable addressed to New York, Sidikou outlined his streamlining plans. They included reducing the force size by at least 750 troops, cutting official travel and reducing rations provided to Congolese soldiers.

In another cable three days later, Sidikou said he intended to ease restrictions on where units can operate, making them more flexible and able to respond to crises.

Election delays and President Joseph Kabila's refusal to step down have fueled violence across Congo, where millions have died in conflicts over the past two decades. An agreement to hold an election by the end of this year is unlikely to hold.

After several years during which the focus had been largely on foreign armed groups, the U.N. mission is contending with increasingly dangerous local rebellions. One, in Kasai, on the border with Angola, has displaced 1.4 million people in a year.

"In the near future, elections-related violence might erupt across the whole of the DRC. Consequently, the Force requires the freedom to deploy its troops in a timely manner, to wherever they may be required," Sidikou's cable said.

(Additional reporting by Michelle Nichols in New York and Jason Patinkin in Addis Ababa; Editing by Peter Cooney and Sonya Hepinstall)